Tryit Editor v2.9

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<!DOCTYPE html>
<html>
<head>
<style>
#myDiv {
    border-style: solid;
}
</style>
</head>
<body>

<div id="myDiv">This is a div element.</div>
<br>
<button type="button" onclick="myFunction()">Change width of the bottom border</button>

<script>
function myFunction() {
    document.getElementById("myDiv").style.borderBottomWidth = "10px";
}
</script>

</body>
</html>